Speech is defined as the ability to communicate thoughts, ideas, or other information by means of sounds that have clear meaning to others.


Sponges are primitive multicellular animals that live in water. All adult sponges are sessile (fixed to one spot), most being attached to hard surfaces such as rocks, corals, or shells.


A star is a hot, roughly spherical ball of gas that shines as a result of nuclear fusion reactions in its core. Stars are one of the fundamental objects in the universe.

Starburst Galaxy

Starburst galaxies are galaxies that are in the process of creating massive formations of stars. This type of galaxy emits large amounts of infrared light created by the explosions that happen during the formation of stars.

Star Cluster

Star clusters are groups of stars close to each other in space that appear to have roughly similar characteristics and, therefore, a common origin. Some of the over 100 billion stars in the Milky Way, our home galaxy, are grouped together in either tight or loose star clusters.


Statistics is that branch of mathematics devoted to the collection, compilation, display, and interpretation of numerical data. The term statistics actually has two quite different meanings.

Steam Engine

A steam engine is a machine that converts the heat energy of steam into mechanical energy. A steam engine passes its steam into a cylinder, where it then pushes a piston back and forth.

Stellar Magnetic Fields

Stellar magnetic fields are an assortment of powerful forces that can be observed at the surfaces of and surrounding stars like the Sun. Astronomers have yet to obtain a complete understanding of the magnetic fields of stars, but they continue to observe their activity in the hopes of understanding their effects on a star's interior makeup, atmosphere, rotation, and future evolution.

Storm Surge

A storm surge is a rise in water level caused by a combination of wind and low atmospheric pressure.


Stress is mental or physical tension brought about by internal or external pressures. The feeling of stress may be mild or severe and it can last a short time or over a longer period.

Subatomic Particles

Subatomic particles are particles that are smaller than an atom. In 1940, the number of subatomic particles known to science could be counted on the fingers of one hand: protons, neutrons, electrons, neutrinos, and positrons.


A submarine is a ship capable of operating underwater. Because its great advantage is its ability to stay hidden, the submarine has developed as a tool of warfare.


Succession is a process of ecological change in which a series of natural communities are established and then replaced over time. Ecologists (scientists who study the relationships of organisms with their living and nonliving environment) generally recognize two kinds of succession, primary succession and secondary succession.

Sudden Infant Death Syndrome (SIDS)

Sudden infant death syndrome (SIDS) is the term used to describe the sudden and unexplained death of an apparently healthy infant. This unpredictable and unpreventable phenomenon is the leading cause of death in babies less than one year old and strikes infants of all ethnic or economic backgrounds.


The Sun, the star at the center of our solar system, is an average-sized, middle-aged star. It is a gas ball made mostly of hydrogen and helium, with a small amount of carbon, nitrogen, oxygen, and trace amounts of heavy metals.


A superconductor is a material that exhibits no resistance to the flow of an electric current. Once a flow of electrons is started in such a material, that flow continues essentially forever.


Ancient astronomers assigned the word nova, Latin for "new," to any bright star that suddenly appeared in the sky. They called an extremely bright new star a supernova.


Surgery is the treatment of disease or injury by cutting into the body to repair or remove the injured or diseased body part. Surgery is usually performed by surgeons in the operating room of a hospital or clinic.

Symbolic Logic

Symbolic logic is the branch of mathematics that makes use of symbols to express logical ideas. This method makes it possible to manipulate ideas mathematically in much the same way that numbers are manipulated.


Taste is one of the five senses through which all animals interpret the world around them. (The other senses are smell, touch, sight, and hearing.) Specifically, taste is the sense for determining the flavor of food and other substances.


A telegraph is any system that transmits encoded information by signal across a distance. Although the word telegraph is usually associated with sending messages by means of an electric current, it was used originally to describe a visual system for sending coded messages.


The word telephone comes from two Greek words, tele, meaning "distant," and phone, meaning "sound." Thus, a telephone is a device for carrying sounds over long distances. Many children are familiar with toy telephones that can be made with two tin cans joined by a taunt string or thin wire.


The telescope is an instrument that gathers light or some other form of electromagnetic radiation (from radio waves to gamma rays) emitted by distant sources. The most common type is the optical telescope, which uses a collection of lenses or mirrors to magnify the visible light emitted by a distant object.


The term television refers to any system for transmitting visual images at a distance. Research on such systems dates back to the 1880s, when German scientist Paul Nipkow (1860–1940) invented a device known as the Nipkow disk.


The concept of temperature has two related, but different, interpretations. On a general level, temperature is associated with the sense of hot and cold.

Thermal Expansion

The term thermal expansion refers to the increase in size of an object as that object is heated. With relatively few exceptions, all objects expand when they are heated and contract when they are cooled.


Thermodynamics is the science that deals with work and heat—and the transformation of one into the other. It is a macroscopic theory, dealing with matter in bulk, disregarding the molecular nature of materials.


A storm is any disturbance in Earth's atmosphere with strong winds accompanied by rain or snow and sometimes thunder and lightning. Storms have a generally positive effect on the environment and on human societies because they are the source of most of the precipitation on which the planet depends.